The present invention relates to selectively plugged honeycomb structures, and in particular, to a method and apparatus for use in selectively sealing cells of a honeycomb structure with a powdered plugging material.
Honeycomb structures having transverse cross-sectional cellular densities ranging between 10 and 300 cells/in2 (about 1.5 to 46.5 cells/cm2), more typically between about 100 and 200 cells/in2 (about 15.5 to 31 cells/cm2) and wall thicknesses of between 0.010 and 0.030 in. (about 0.25 to 0.76 mm), especially when formed from ceramic materials are used as solid particulate filter bodies, and require selected cell channels of the structure to be closed or plugged at one or both of their ends.
It is well known that a solid particulate filter body, such as a diesel particulate filter, may be formed by a matrix of intersecting, thin, porous walls which extend across and between two of its opposing end faces and form a large number of adjoining hollow passages or cell channels which also extend between and are open at the end faces of the structure. To form a filter, one end of each of the cell channels is closed, a first subset of cell channels being closed at one end face and the remaining cell channels being closed at the remaining opposing end face of the structure. Either of the end faces may be used as the inlet face of the resulting filter. The contaminated fluid is brought under pressure to the inlet face and enters the body via those cells which have an open end at the inlet face. Because these cell channels are closed at the outlet end face of the body, the contaminated fluid is forced through the thin, porous walls into adjoining cells which are sealed at the inlet face and open at the outlet face of the filter body. The solid particulate contaminant in the fluid which is too large to pass through the porous openings in the walls is left behind and a cleansed fluid exits the filter body through the outlet cell channels, for use.
Up to this time selected cells were sealed or plugged with a foam-type cement, as disclosed in U.S. Pat. Nos. 4,297,140, 4,239,357 and 4,559,193. The cement is formed into a paste by mixing ceramic raw material with an aqueous binder, such as methyl cellulose, plasticizer and water. When using this foam-type cement, both ends of the honeycomb structure are covered with flexible masks having holes through which the cement is pushed into the ends of the cells. There are numerous disadvantages associated with this type of filling or plugging material. The masks must be cleaned and dried after each use. Unclean masks can cause missing plugs requiring additional manual labor. The cement batch is time, shear and temperature dependent; often thrown out, unused due to age restrictions; and, drying is required to remove the water content.
The process of the invention is provided to overcome the disadvantages of prior art plugging methods. The process is particularly suited for using only one mask and charging a powdered plugging material into selected cells of a honeycomb structure for later use in diesel particulate filters. Powdered plugging materialxe2x80x9d as used in the description of the present invention means a material which is formed of particulates. This type of plugging material is in contrast to current prior art plugging materials which are a cement, having a wet, paste-like consistency. Although any powdered plugging material suitable for forming plugs may be used in the present invention, the preferred material comprises particulate sealant as described and claimed in co-pending patent application filed concurrently with the present application in the names of by M. Fabian, S. Lakhwani, and M. Roberts under the title PARTICULATE SEALANT FOR FILTER PLUG FORMING.
In the process of the present invention, a ceramic honeycomb filter body is fabricated from a honeycomb structure having a plurality of cell channels extending therethrough which are arranged in mutually parallel rows and in mutually parallel columns at its end faces. A first end face of the honeycomb structure is covered with a mask having openings corresponding to open ends of a first plurality of cell channels. Powdered plugging material is loaded through the mask openings into the first plurality of cell channels at open cell ends adjacent the first end face. The plugging material being composed of a multiplicity of particulates passes or flows the length of the cell channel to the remaining cell ends opposing the first end face and adjacent the second end face. In the process of forming plugs thereat, the powdered plugging material is compacted by suitable compaction means such as vibration, centrifuging, manual compaction and the like, and then heated to a temperature sufficient to form a flowable material which fills and adheres to all four walls of the cells. Upon cooling the flowable material solidifies to form plugs at the cell ends.
Thereafter, the mask is removed from the first end face, and the plugging process is repeated for the second end face. However, a mask need not be provided for the second end face. The so-plugged cell channels at the second end face effectively function to form a mask for plugging the first end face. Powdered plugging material is loaded into the open ends of a second plurality of cell channels, the second plurality of cell channels corresponding to open cell channels at the second end face of the honeycomb structure; and, plugs are formed as before at the ends of cell channels opposing the first end face of the honeycomb structure.
In a preferred embodiment the step of covering the first end face of the honeycomb structure includes applying a solid transparent film across and onto the first end face of the honeycomb structure; scanning the film to locate the first plurality of cell channels; generating in response to the scanning step a first set of signals indicating the location of the first plurality of cell channels; generating in response to the first set of signals a second set of signals for positioning an opening forming tool, i.e., a laser, with respect to the first plurality of cell channels; and, activating the opening forming tool in response to the second set of signals whereby openings are formed through the film corresponding to the first plurality of selected cell channels.